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Relation of Speed and Speed Limits to Crashes National Forum on Speeding Washington, D.C., June 15, 2005 Susan Ferguson, Ph.D.

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Presentation on theme: "Relation of Speed and Speed Limits to Crashes National Forum on Speeding Washington, D.C., June 15, 2005 Susan Ferguson, Ph.D."— Presentation transcript:

1 Relation of Speed and Speed Limits to Crashes National Forum on Speeding Washington, D.C., June 15, 2005 Susan Ferguson, Ph.D.

2 IIHS Overview  Relation of speeds to crashes  Relation of speed to crash severity  Relation of speed limit changes to speed and crashes  Characteristics of excessive speeders

3 IIHS Relation of speed to crashes Solomon 1964, Cirillo 1968  Solomon studied speeds of crash-involved vehicles and overall traffic speeds on rural highways  Cirillo conducted a similar analysis for daytime crashes occurring on interstates; crash types limited to rear-end, angle, and same-direction side-swipes

4 IIHS Findings  Both studies found a U-shaped relationship between vehicle speed and crash incidence  Crash rates were lowest for drivers traveling near the mean speed, and increased with deviations above and below the mean  Low-speed drivers were more likely to be involved in crashes than relatively high speed drivers

5 IIHS U-shaped curve- Solomon/Cirillo Cirillo Solomon

6 IIHS Explaining Solomon’s and Cirillo’s findings  Solomon: –many crashes occurred at intersections and involved stopped or slowing vehicles –data were collected on 1950s era roads, which lacked turn lanes and passing lanes  Cirillo: –many crashes, especially rear-end collisions, may have been related to traffic congestion, which by definition involves stopped or slowing vehicles

7 IIHS High Crash Rates at Low Speeds(<40mph) Crash Rates constant (40-68mph) High Crash Rates (>70mph) Combining all we have this U shaped curve Explaining the U-Shape Injuries increase with speed Risk of Injury Crashes and Speed, Cowley 1987

8 IIHS West and Dunn, 1971  Data collected on a state highway in Indiana with speed limits of mph  Found a U-shaped relationship like some of the earlier studies  Crash risk was greatest for vehicles traveling more than two standard deviations above the mean speed  Does not explain significant percent of speeding-related single vehicle crashes

9 IIHS West and Dunn speed curve Likelihood of being involved in crash flat within 15 mph of mean speed Replicated U shape like previous studies Deviation from mean speed, mph

10 IIHS Kloeden et al., 1997  Risk of being involved in an injury crash was lowest for vehicles traveling near or below the median speed and increased exponentially at higher speeds Risk increases exponentially

11 IIHS Shinar, TRB Special Report 254 (1998) On the relation of speeds to crashes  There is sufficient evidence to indicate a driver’s speed is a correlate of crash involvement  Speed deviation of crash-involved vehicles from the average appears to be positively related to crash probability, especially for arterial highways and interstates  The principal factor…is the requirement to slow down to make turns and to enter and exit high-speed roads

12 IIHS Relation of speed to crash severity  The relationship between vehicle speed and crash severity is unequivocal and based on the laws of physics  Velocity change in a crash ( Δ V) is a critical measure of crash severity

13 IIHS Speed and crash severity (continued)  The likelihood of being injured, and injury severity in a crash, depend on the change in speed ( Δ V) at impact (Bowie and Waltz,1994) As ΔV increases injury risk also increases

14 IIHS Speed and crash severity (continued)  O'Day and Flora (1982) and Joksch (1993) found that the risk of a car driver being killed in a crash increased with increases in speed Speed Increase Risk Increase Exponential

15 IIHS Relation of Speed Limit Changes to Speeds and Crashes

16 IIHS 1974 National Maximum Speed Limit (NMSL) legislation  In 1974, Congress established the NMSL of 55 mph –mainly implemented to conserve fuel –traffic fatalities declined 16 percent, from 54,052 in 1973 to 45,196 in 1974

17 IIHS US DOT study  Joint NHTSA-FHWA task force studied safety effects of the NMSL (1980) –while the precise number of lives saved is unknown, an estimated 20,000 to 30,000 lives were saved by the NMSL from

18 IIHS TRB Special Report Concluded: The lower limit [of the NMSL] reduced both travel speeds and fatalities, although driver speed compliance gradually eroded

19 IIHS Partial repeal of the NMSL in 1987  States granted authority to raise speed limits to 65 mph on rural interstates

20 IIHS Effects of the 1987 speed limit changes Speed  McKnight et al.,1989: 65 mph states, 48% increase in drivers exceeding 65 mph; 55 mph states, 18 % increase  NHTSA, 1992: –3 mph increase in average speeds –4 mph increase in 85 th percentile speeds –0.7 mph increase in standard deviation

21 IIHS Effects of the 1987 speed limit changes Crashes  Garber and Graham,1989: –15% increase in fatalities on rural interstates – 5% increase in fatalities on rural non-interstates  McKnight et al., 1989: –22% increase in fatal crashes on rural interstates – 1% increase in fatal crashes on 55mph roads

22 IIHS Effects of the 1987 speed limit changes Crashes (continued)  Baum et al.,1991: –19% increase in fatalities on rural interstates relative to other rural roads  NHTSA 1992: –No increase in fatality rates on rural interstates –12% decrease in fatality rates on rural interstates in states that retained 55 mph

23 IIHS Effects of the 1987 speed limit changes: Crashes (continued)  Lave and Elias,1994: –reported a 3 to 5 percent decrease in statewide fatality rates in states that raised speed limits to 65 mph

24 IIHS Full repeal of the NMSL in 1995  States granted full authority to establish speed limits on all roads

25 IIHS Effects of full repeal of the NMSL Interstate and freeway speeds  Retting and Green, 1997: –mean speeds and 85 th percentile speeds increased by 2 to 5 mph –proportion of cars exceeding 70 mph increased by 15 to 50 percent –speed variation, as measured by the standard deviation, increased by 5 to 15%

26 IIHS Effects of full repeal of the NMSL: Crashes  Farmer et al., 1999: –15% increase in fatalities and 17% increase in fatality rates on interstates with increased speed limits –no significant change on non-interstate roads  Frith et al., 2002: –35% increase in fatalities in 70 mph states –38% increase in fatalities in 75 mph states

27 IIHS Effects of full repeal of the NMSL Crashes (continued)  NHTSA,1998: –6% increase in fatalities and 15% increase in injuries on interstates –2% decrease in fatalities and 3% increase in injuries on non-interstate roads

28 IIHS Effects of full repeal of the NMSL Crashes (continued)  McCarthy (TRB Report 254),1998: –positive relationship between crash severity and speed dispersion for rural interstates –speed dispersion is very important for fast drivers  Cato Institute, 1999: –raising speed limits reduced injuries significantly, increased fatalities insignificantly and had a considerable net cost benefit

29 IIHS Conclusions  Overall crash involvement as a function of travel speed generally follows a U-shaped curve  Lower than average traffic speeds are unavoidable due to traffic conditions and vehicle maneuvers that require low speeds  Higher than average speeds generally violate traffic laws and can be addressed through consistent law enforcement

30 IIHS Conclusions (continued)  Higher and lower than average speeds don’t have equal consequences; as crash speeds increase, so does crash severity  The overwhelming majority of evidence suggests that reductions in speed limits reduce vehicle speeds and crashes; increases in speed limits increase speed, as well as crashes  Excessive speeders are more likely to be male, younger, and to have poor driving records


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